278 



IMPROVEMENT OF THE SOIL. 



in twelve years. {Liebig.) The excrements of different 

 plants require different periods to effect their conversion into 

 humus ; the excrements of flax, peas and clover, for example, 

 when grown on argillaceous soils, require the longest period 

 to effect this change. 



From the views now presented, we may see the reason 

 why the interchange of crops produces effects so highly bene- 

 ficial. It is because the cultivation of different kinds of 

 plants on the same field, enables each to extract certain com- 

 ponents of the soil, which are necessary to it, and to leave 

 behind or restore those which a second or third species may 

 require for its growth, and perfect development. In con- 

 structing a system of rotation, therefore, we must have refer- 

 ence to the structure of plants, to the alkalies and salts 

 which each species of plant requires, and to the matters 

 which they excrete from their roots. We will therefore con- 

 clude this subject with a series of rules derived both from ex- 

 perience, and from the views now presented. 



1. Two exhausting crops should not succeed each other on 

 the same field, because their structure is similar, and they de- 

 rive similar ingredients from the soil. 



2. Culmiferous, leguminous and root crops should alternate 

 with each other, because their structure, composition and ex- 

 cretions are most diverse, and the least injurious to each 

 other. If the first crop is a hoed crop, the second should be 

 a grain crop ; although two hoed crops such as corn and po- 

 tatoes, or turnips, are better than two grain crops. 



3. A grain crop should succeed a hoed crop, rather than 

 precede it. The reason in this case appears to be, that the 

 manures can be more perfectly worked into the soil by a hoed 

 crop, and the soil is left in a better condition for grain. 

 There are, however, two exceptions to this rule. 1. When 

 clover makes one crop in the rotation, it is found that wheat 

 may succeed it with advantage, because they require different 

 alkalies or salts, and the roots of the clover prepare the soil for 



